The present invention generally relates to wireless communication networks, and particularly relates to improving communication performance in wireless communication networks employing Automatic Repeat Request (ARQ) retransmission mechanisms, such as the Hybrid ARQ (H-ARQ) mechanism used in cdma2000-based wireless communication networks.
Limiting per-user interference levels in wireless communication networks employing Code Division Multiple Access (CDMA) increases the number of users that can be supported simultaneously, and improves reception conditions for all users. Maintaining each user's reverse link transmit power to a minimum level needed for satisfactory communication performance stands as a primary limiting mechanism, and CDMA-based networks commonly rely on so-called outer-loop and inner-loop power controls to control the transmit power used on various forward and reverse link radio channels.
As an example, the reverse link data signal from a given user (mobile station) is received at a network base station and its received signal strength is compared to an outer loop power control target. If the signal strength is above the target, the base station sends a “down” command to the user, or sends an “up” command if the signal strength is below the target. The comparisons and up/down command transmissions occur at a defined inner loop power control rate, which usually is many times per second. The outer loop control target usually is adjusted at some slower outer loop power control rate based on a measured or estimated Frame Error Rate (FER) determined for the received signal.
Applying ARQ mechanisms within the above context offers the opportunity to further minimize each user's transmit power. For example, recent revisions of the IS-2000 standards for CDMA2000 wireless communication networks introduced higher-rate reverse link packet data channels employing H-ARQ. A data transmission based on H-ARQ comprises whatever number of transmissions of a given data packet are needed for successful receipt, but usually subject to some limit on the number of retransmissions. More particularly, the H-ARQ method adopted for the Reverse Packet Data Channels (R-PDCHs) used in later revisions of the IS-2000 standards use multiple transmissions that yield gradually decreasing statistical reception error rates.
For example, a H-ARQ control mechanism may define a three-try transmission sequence having an ending or cumulative target FER of 1%. A given data packet (or sub packet) might be transmitted from a mobile station according to a three-try ARQ transmission sequence that uses different target FERs for the different subpacket transmissions in the sequence, such that the overall FER for all subpackets terminated at the first, second, or third ARQ transmissions is 1%. (Other FER calculations can be used, of course.) Generally the FER target used for the first and/or second subpackets is higher with the advantage being that the target signal strengths required for these transmissions are relatively low compared to a non-ARQ single transmission target of 1%. Thus, to the extent that the second or third transmissions in the defined ARQ transmission sequence are not always needed for successful data reception, the overall transmit power of the mobile station can be kept lower than would be needed absent the use of ARQ.
The use of ARQ carries with it certain disadvantages as well. For example, because high FERs and/or lower transmit power typically are used at least for the initial sub packet transmission by a mobile station, the likelihood is that the transmission will not be received correctly, requiring one or more retransmissions of that same data. Transmitting the same data multiple times lowers the effective reverse link data rate. Indeed, an average of 2.2 ARQ transmissions per sub packet have been observed for CDMA2000 systems in exemplary simulation environments, meaning that a peak data rate of 1.8 Mbps more typically yields a lower effective data rate of about 818 Kbps (1.8 Mbps/2.2 transmissions).